1. Field of the Invention
The invention is generally related to the production of foamed polymeric materials and, more particularly, to the use of a chemical reaction between an amine and a carbonyl which produces a ketimine, or aldimine, and water byproduct as an in-situ mechanism for foaming polymeric materials.
2. Description of the Prior Art
Production of foamed polymeric materials has become increasingly important to meet the demands of modern living. The foamed polymeric materials are widely used for insulation purposes in aircraft, automobiles and other land vehicles, and in buildings. In addition, foamed polymeric materials have utility as filter media, as catalyst supports, in biological implants for drug delivery, and in a wide variety of other applications. Polyarylene ether ketones (PAEKs) have strength and toughness characteristics which make them an attractive substitute for metal and steel as structural support members. Foamed PAEKs have the advantage of being a light weight, high strength material. By sandwiching a PAEK foam between non-foamed PAEK layers, a rigid, lightweight structure is produced that does not suffer from interfacial incompatibility between the foam and the non-foamed members. It would be advantageous to provide a low cost, environmentally sound, technique for foaming polymeric materials, particularly high strength materials such as PAEKs and high performance materials such as polyimides.
U.S. Pat. No. 4,026,833 to D'Aleio discloses the production of foamed polyimides being produced via a chemical reaction between aromatic polyisocyanates and aromatic dianhydrides. During the reaction, carbon dioxide is produced and serves as a blowing agent to create the foam. U.S. Patent to Parfoundry et al. is another example of using carbon dioxide as a blowing agent for creating a foamed polymer. In Parfoundry et al., ketimines are used as isocyanate reactive compounds in the production of polyurethane foams.
U.S. Pat. No. 4,990,548 to Gillis et al. discloses the use of a ketimine reagent to produce a foamed polymeric product. In Gillis et al., a ketimine product is created by the reaction of an amine and a carbonyl compound. A gaseous carbonyl product is then generated when water reacts with the imine, and the gaseous carbonyl product serves as a blowing agent for foaming the polymer.
A water foamed poly(aryl ether) (PAE) is described in U.S. Pat. No. 5,179,130 to Bland et al. In operation, water is blended with the PAE in the melt state under pressure. Subsequently, the pressure is released and the gaseous water acts as a blowing agent to foam the PAE. U.S. Pat. No. 4,506,037 to Suzuki also discloses the use of water as a blowing agent. In Suzuki, water is adhered to microporous particles which are then melt blended with a polymer under pressure. Subsequent depressurization at elevated temperatures is then used to foam the system. U.S. Pat. No. 5,153,232 to Primeaux discloses a system where the claim water is "the sole blowing agent" for the production of polyurea foams. However, in Primeaux the gas which actually "blows" the foam is carbon dioxide that is produced from a reaction between water and isocyanate.
U.S. Pat. No. 4,426,463 to Gagliani and U.S. Pat. No. 4,546,115 to Long disclose the use of both water and alcohols, in combination, as blowing agents. In Gagliani, a monoimide is formed by the reaction of an oxoimine with an aromatic anhydride which is subsequently esterified, and is followed by the addition of a diamine. The reaction product is probably an amide-ester. The dried product is heated to produce an imide, with a consequent loss of the esterifying alcohol and water, both in a gaseous state, to foam the molten mass. In Long, a dianhydride is esterified with subsequent addition of diamine and drying of the product. Heating produces water and alcohol which results in a foamed imide.
The above-identified patents have the advantage that organic blowing agents, which have been commonly used in prior art foam production, are not employed. Organic blowing agents pose disposal and explosion problems. Hence, the production of water vapor or carbon dioxide offers a cleaner, as well as safer, means for producing foam. In addition, the above-identified patents generally describe foaming techniques that do not require excessive mechanical manipulation of the polymeric material to foam the product. Thus, the cost for foam production is reduced due to lower equipment and energy costs.
Nevertheless, none of the above-identified methods disclose an ideal technique for foaming PAEK materials. Thus, a need exists for PAEK foam production that is low cost, effective, and does not pose environmental hazards. In addition, it would be advantageous to provide an alternative technique for foaming a wide variety of polymeric materials in a low cost, effective, and safe manner.
U.S. Pat. No. 4,721,732 to Dubrow et al. discloses the production of microporous PAEK materials by a blending and extraction process. In Dubrow et al., a blend of two molecularly compatible polymers is formed with subsequent selective solvent extraction of one polymer to produce the microporous structure. U.S. Pat. Nos. 4,904,426, 4,992,485, and 5,064,580 all disclose similar processes of mixing and "leaching" to produce microporous PAEK fibers or films. U.S. Pat. No. 4,904,426 to Lundgard et al. achieves the microporous structure by mixing PAEKs with an organic solvent, extrusion of the mix, and subsequent solvent extraction. U.S. Pat. No. 4,992,485 to Koo et al. describes the production of microporous poly(ether ether ketone) (PEEK), which is a specific type of PAEK, using non-sulfonating acid solvents. In Koo et al., a coagulation step is used instead of extrusion before solvent extraction. U.S. Pat. No. 5,064,580 to Beck et al. discloses the use of a plasticizer instead of a solvent for producing a microporous structure.
It should be understood that the microporous structures discussed in the above-identified patents are not true "foams". Rather, these structures are generally solid and have micropores therein. A process for producing foamed PAEK materials is needed. A foamed PAEK material will have the advantage of being lightweight, as well as the inherent strength and toughness properties of PAEKs.